1. Field of the Invention
The present invention generally relates to the art of object oriented programming in the field of computer science and, more particularly, to a technique for connecting objects on different pages of an visual application program builder which facilitates the design of an application.
2. Description of the Prior Art
The field of computer science known as object oriented programming (OOP) is now the preferred environment for building user-friendly, intelligent computer software. Key elements of OOP are dam encapsulation, inheritance and polymorphism. These elements may be used to generate a graphical user interface (GUI), typically characterized by a windowing environment having icons, mouse cursors and menus. While these three key elements are common to OOP languages, most OOP languages implement the three key elements differently.
Examples of OOP languages are Smalltalk, Object Pascal and C++. Smalltalk is actually more than a language; it might more accurately be characterized as a programming environment. Smalltalk was developed in the Learning Research Group at Xerox's Palo Alto Research Center (PARC) in the early 1970s. In Smalltalk, a message is sent to an object to evaluate the object itself. Messages perform a task similar to that of function calls in conventional programming languages. The programmer does not need to be concerned with the type of data; rather, the programmer need only be concerned with creating the right order of a message and using the right message. Object Pascal is the language used for Apple's Macintosh.RTM. computers. Apple developed Object Pascal with the collaboration of Niklaus Wirth, the designer of Pascal. C++ was developed by Bjarne Stroustrup at the AT&T Bell Laboratories in 1983 as an extension of C, which is the language in which the Unix.RTM. operating system is written. The key concept of C++ is class, which is a user-defined type. Classes provide object oriented programming features and typically has two kinds of clients called instances and subclasses. C++ modules are compatible with C modules and can be linked freely so that existing C libraries may be used with C++ programs. The preferred embodiment of the invention is written in C++.
The most widely used object based and object oriented programming languages trace their heritage to Simula developed in the 1960s by O-J. Dahl, B. Myhrhaug and K. Nygard of Norway. Further information on the subject of Object Oriented Programming may be had by reference to Object Oriented Design with Applications by Grady Booch, The Benjimin/Cummings Publishing Co., Inc., Redwood City, Calif. (1991), and An Introduction to Object-Oriented Programming by Timothy Budd, Addison-Wesley Publishing Co. (1991).
Programs have been developed to aid programmers in writing application programs. An early example of such a program in the object oriented programming environment is MacApp for the Apple Macintosh.RTM. computers. MacApp is an application framework which is basically a complete, self-contained application that implements most of the Macintosh.RTM. user-interface, including pull-down menus and scrolling windows. The program allows the programmer to add, modify or override objects to customize the framework for a specific application. This is all done interactively using a GUI. For more information on the MacApp framework, see "MACAPP: AN APPLICATION FRAMEWORK" by Kurt Schmucker, BYTE Magazine, August 1986, pp. 189 et seq.
MacApp is but one of many visual program builders. Typically, when a visual program builder is started by double clicking on the ICON representing the builder program, a visual builder toolbox window is opened. The visual builder toolbox window contains a tool box notebook, each page of which contains the GUI objects, objects created by using the visual builder, or foreign objects that the user receives from other resources. The toolbox notebook is stored by using a persistent framework.
When building application programs with a visual program builder, it is often desired to connect two visual objects. For example, a button or slider object may have a text object associated with it such that when the button or slider is manipulated by a user with a mouse cursor, the text will be displayed. In a linking mode, if both source and target objects are on the same notebook builder page, then the user just presses the left mouse button down while the pointer cursor is in the source object and then moves the pointer cursor to the target object and releases the mouse button. During the mouse movement, a rubber banding line is drawn from the starting point of the source object to the pointer cursor position. After the mouse button is released, the link will be replaced by a link with arrow tail and arrow head to indicate the direction of the link.
A problem arises, however, when two objects on different notebook pages are to be connected. In the visual application builder environment, only one notebook page can be seen at any time. In order to connect the source and target object, both objects should be visible at the same time.